Composition blackboard and method of making same



Patented Apr. 29,..1930 I i v 4 1,756,393

UNITED STATES FOSTER DEE SNELL, OF BROOKLYN, NEW YORK COMPOSITIONBLACKBOARZD AND METHOD OF MAKING SAME Ll'o Drawing. Application filedDecember 23, 1927. Serial No. 242,298.

This invention relates to composition blackconverted into slabs orsheets of suitable boards and methods of making same; and it shape andsize by the ordinary glass making comprises as a new article a hardopaque slab and working methods, and which is, so to or sheet composedof a body of fine grained speak, rough all through so that the surface 5opaque dark colored. mineral matter such as cannot wear smooth; nor canlocal smooth 55 chromite, magnetite, rutile, menaccanite, areas form.The composition may be reetc., with a vitreous binder of the nature ofgarded either as a glass body containing a ordinary glass, such binderbeing advanta-- large amount of disseminated uniformly disgeously alsodark colored or opaque and said tributed granules of moderate size andmodslab or sheet having sufficient superficial crate hardness; or as abody of such granules 60 roughness or'tooth to make writing therebondedby a fused glass matrix. In my inon convenient; and it also comprisesai-methvention I incorporate into an ordinary sodaod of making such anarticle wherein a glass lime glass composition as much of a finecomposition is charged with as much fine, grained, opaque, dark coloredmineral matdark opaque chemically inert mineral matter ter as the glasswill take up without unduly 65 as it will take up without forfeitingfluidity forfeiting fluidity at glass working temperain a molten state,the fused mass is converted tures and 1 under glass working conditions.into a slab or sheet and an outer surface of It is my object to producea composition the slab or sheet is removed to give a superwhich can bemade and handled in an ordificial matt appearance; all as more fullyherenary glass factory. The mineral matter used inafter set forth and asclaimed. should be rather harder than the glass so that Blackboards maybe, and sometimes are, in the blackboard preferential wear takes merelypainted wood, or wood covered with place in the binder between grains.It should a surfacing composition. These however are be tolerably fine,but not of colloidal fineunsatisfactory in use and for schools and theness since its function is to ofier a rough sur- 75 like, slate slabs orsheets are frequently used. face to the chalk or crayon.- A finenesssuch It is a matter of some difiiculty to obtain unithat the mineralmatter will all pass through t ary slabs, of any size desired'and toprovide 2/100 mesh sieve is about rightfor the pres sheets of uniformthickness while the damage ent purpose. Since I am using the glass notin transportation and handling is consideronly as a binder but as aconvenience in work- 80 able. Nor is a slate surface usually wholly inginto slabs or sheets, I do not wish to disuniform or of the exactcharacter desired for turb its ordinary properties and therefore I usewith chalk so these slate sheets are freuse mineral matters which arechemically quently further treated, being rubbed, sandinert with regardto the particular glass comed, coated or painted. These coatings are notposition used. The best of these minerals I long lived. Ghalks andcrayons usually conhave found is chromite, or chrome iron ore, tain moreor less hard granular matter, such which has the right amount ofhardness, is as sand, and have considerable abrading efopaque and darkcolored, has substantially fect, wearing through these coatings in time.no chemical action on ordinary glass compo- 40 Various substitutes forslate have been pro- .sitions although readily wet and bonded posed buthave not met with any great com- I thereby; and is readily distributedthrough mercial success. Black plate glass is one of molten" glasswithout segregation. It can these which has been-used to a limitedexeither be admixed with molten glass or be tent; the glass being givena surface adapted admixed with powdered glass in the cold and to take,chalk by sand blasting. Unfortunatethe mixture subsequently heated tomelt the ly, the roughness of the surface soon wears glass. lts hardness(5.5 on Mphs scale) is 7 down, leaving local smooth glass areas whichabout r1 ht in giving the desired hardness will not take chalk. Jdiflerential with glass which generally has a In the present invention Ihave provided hardness around 2.5. lthagnetrte may be used.

a composition for blackboards which may be It has a su table hardnessand its opacity and blackness are good. But it is somewhat more solublein glass to which it gives up FeO, thereby changing the glasscomposition and properties somewhat. The extent of this 5 attack ishowever not sufficient to preclude its convenient use and to the extentthat iron dissolves, the glass is darkened which is desirable. Rutile ornative TiO has a proper hardness and inertness but is not as opaque ordark as the two minerals just mentioned.

Reducing conditions aid attack but also produce a dark purple color inthe bonding glass. Menaccanite, ilmenite and various other dark coloredminerals may also be used. I do not find emery or other forms of aluminasuitable for my purposes even when these I minerals are black and opaquesince the granule hardness is rather too great for blackboard purposesand further, thealumina acts chemically on glass with production ofalumino-silicates and interference with its ordinary properties.

The glass employed as a bond may be any ordinary glass composition usedfor making sheet and plate glass. Ordinary soda-lime glass works veryWell. While white glass compositions may be employed I find in practicethat it is well to color the glass as it gives a better blackboard.Copper oxids, manga- 0 nese oxids, cobalt oxids and various othercolorin agents are suitable. In practice, I ordinarily employ MnO inquantities sulficient to give a dark or opaque amethystine color. A goodglass for the present purpose may be made with the aid of 100 partssilica, 35 parts soda ash, 12 parts lime, 10 parts Chile saltpeter and10 parts pyrolusite; this giving about 140445 parts of molten glass. hiscomposition in a molten state will take u about half its weight of added100 mesh 0 romite Without undue loss of liquidity and malleability atglass working temperatures. The opaque mineral used may be groundtogether with the proper amount of glass or with such a composition asjust stated and the whole fused. This is more convenient in glass housepractice than working the mineral matter into the molten glass. About 60parts of chromite passing 100 mesh standard sieve give the desiredresults with a batch of glass made of the indicatedsubstances in theindicated pro ortions. In grinding chromite so that it Wlll all justpass through 100 mesh sieve much of the material is much finer; but withthis grinding the right proportions of grain sizes are obtained. Coarsermaterial gives a blackboard which is too rough while with chromitereduced to an impalpable powder and worked into glass the product is toomuch of the nature of a colloidal dispersion;

with it I do not get the "kind of surface I Want.

Fused glass'containing dispersed opacifying material made under thepresent invention is formed into a sheet or slab *in any of the ordinaryways, such as the rolling operation ordinarily employed in making plateglass. With chromite present in the proportions specified and of thecharacter specified, rolling and other operations are practically asready as with ordinary plate glass. I The sheet or slab on cooling issmooth surfaced and glossy and to put it in a condition for use,- theouter surface must be removed to expose the distributed mineral matter.This may be done by sand blasting or by pickling with hydrofluoricacid.- Once formed, the rough and dull surface persists indefinitelylong, taking chalk or crayon marks readily and not wearing to givesmooth spots or areas. The granular matter being a little harder thanthe glass, the initial type of surface persists however long the boardmay be in use.

What I claim is:

1. A composition of matter adapted for use as a blackboard andcomprising a vitreous matrix having uniformly distributedtherethrough'an opaque, inert and finely divided, somewhat hardergranular mineral.

2. A composition of matter adapted for 90 use as a blackboard andcomprising a matrix of glass having uniformlydistributed therethrough anopaque, inert somewhat harder granular mineral, said mineral beingcomposed of grains of various sizes, the mixture passing through astandard mesh sieve.

3. A blackboard comprising a matrix of colored glass having'uniformlydistributed therethrough inert and finely divided opaque,- dark coloredmineral material, in quantity 10o sufficient to impart a grainy textureto the mixture.

4. A composition of matter adapted for use inpermanent blackboards andthe like, said composition comprising a vitreous matrix, and finelydivided chromite uniformly distributed therethrough in quantitysuflicient to give a grainy texture, said chromite being composed ofgrains passing a 100 mesh standard sieve.

5. In the manufacture of compositions adapted for use as blackboards andfor simi lar purposes the process which comprises distributingmoderately fine, moderately hard grains of opaque dark colored mineralmatter throughout a body of glass material, melting the mixture toglass, forming such glass into a sheet and removing the outer surface.

6. In the manufacture of compositions adapted for use as blackboards andfor simi-' lar purposes the p'ocess which comprises distributingmoderately fine grains of chromite throughout a body of glass material,melting the mixture, forming the mixture into a solid sheet and removingthe outer surface.

7. A composition of matter adapted for use as a blackboard andcomprising a vitreous matrix having uniformly distributed therethroughan inert and finely divided somewhat granular mineral, the said vitreous130 ing' a sandblasted surface developing the dispersed mineralmatgraininess due to such ter.

In testimony whereof, I have hereunto affixed my signature.

FOSTER DEE SNELL.

